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You can do as much purchasing as you like; each time a card is purchased, you draw a new one from the deck, which is available to purchase immediately. If you have somehow saved up 50 energy, you could buy many, many cards. There is no limit to how many cards you are allowed to own, but if you're collecting that much energy, somebody else has probably won already.

If you hate the cards that are available, you may spend 2 Energy to sweep all three available cards away into the discard pile, and replace them with new cards from the deck. You may do this repeatedly, at 2 Energy per time you sweep the cards, and you can do this both before and after buying a card. (You don't actually need to buy a card, either; if you only have 2 Energy, and your opponent is staring at a card you don't want him to have, you can sweep the cards without buying anything.)

(The above bits were paraphrased from the rules, under step three of your turn, "Buying Cards".)